This invention relates to position control and, more particularly, to a system for controlling the position of a movable member and a motor drive therefor.
One commonly used technique to control the position of a movable member, i.e., a load, utilizes a direct current motor incorporated in a feedback loop to drive the load. A transducer senses the actual position of the load. The output of the transducer is applied to a summing junction with a command signal representative of the desired position of the load. The difference signal generated by the summing junction feeds the motor such that the motor drives the load to the desired position.
Another commonly used technique to control the position of a load is to use a stepper motor to drive the load. Depending upon the desired degree of reliability, the stepper motor is incorporated into a feedback loop or operated open loop.
By comparison to direct current motors and stepper motors, an induction motor is economical, efficient, and rugged. Known techniques for controlling the position of an induction motor, however, have not been as precise as direct current and stepper motors. These techniques employ a mechanical limit switch together with a friction brake to stop an induction motor at a predetermined load position with an accuracy in the order of .+-.0.250 of an inch to .+-.0.100 of an inch. At the cost of additional complexity, the accuracy can be improved by slowing the motor down in stages prior to the mechanical braking.
Although it is known to brake an induction motor electronically by injecting direct current into its stator windings, such electronic braking, has in the past been initiated responsive to the disconnection of power from the motor rather than in response to load position. As a result, control over the position at which the load stops has not been exercised by prior electronic techniques for braking induction motors.
My U.S. Pat. No. 4,242,621, issued Dec. 30, 1980, discloses a programmable limit switch that permits one or more operations to be controlled responsive to the position of a movable member, specifically a machine tool slide. A transducer senses the actual position of the machine tool slide. Binary signals representative of sets of positional upper and lower limits corresponding to each of the operations are stored in an addressable memory. Each of the limits in the memory is sequentially compared with the actual position of the machine tool slide and an output switch is energized while the machine tool slide is between the limits set for an operation, thereby executing such operation. The limits within which an operation is performed can be programmed simply by changing the signals representing such limits in the addressable memory.